Ion transfer mediated by TEMPO in ionophore-doped thin films for multi-ion sensing by cyclic voltammetry

We report here on the development of thin-layer ion-selective membranes containing lipophilic TEMPO as a phase-transfer redox mediator for the simultaneous detection of non-redoxactive ions. This redox probe was recently introduced by our group and provides ideal ion-transfer waves when the membrane is interrogated by cyclic voltammetry. To perform multianalyte detection in the same sensing film, plasticized PVC-based membranes were doped with lithium and potassium ionophores in addition to a lipophilic cation-exchanger. The ionophores allow for ion discrimination owing to the different ionophore-cation complexation constants and the oxidation of TEMPO to the oxoammonium form results in the selective transfer of lithium and potassium at different potentials. The resulting voltammograms have half-peak widths of 100 and 102 mV, and the peak separation between anodic and cathodic scans is 8 and 9 mV for lithium and potassium, respectively, close to theoretical expectations. High peak resolution was observed, and the ion-transfer waves are still distinguishable when the ion activities differ by three orders of magnitude. These parameters are remarkably better than those obtained with other redox probes, which is important for multianalyte detection in the same voltammetric scan. Optimized membranes showed independent Nernstian shifts (slopes of 59.23 mV and 54.8 mV for K⁺ and Li⁺, respectively) of the peak position for increasing ion concentrations. An idealized model for two ionophore-based membranes combining redox and phase-boundary potentials was applied to the proposed system with excellent correlation. Potassium and lithium ions were simultaneously detected in undiluted human serum samples with good accuracy and precision.